The search for compositions which have a combination of excellent insecticidal activity and desirable low toxicity to plants and mammals is a continuing one because of factors such as the desire for compounds exhibiting greater insecticidal activity, better selectivity, low environmental impact, low production cost and effectiveness against insects resistant to many known insecticides.
Various parts of the neem (or nim) tree have long been used in India for their reputed medicinal or insecticidal properties. This subtropical tree is native to the arid regions of India, Pakistan, Sri Lanka and parts of Southeast Asia and Africa.
Although all parts of the neem tree appear to have natural resistance to pests and diseases, the seeds appear to have the greatest resistance. Formulations and extracts of the seeds have been shown to be effective against many species of crop pests including gypsy moths, Japanese beetles, aphids, tobacco budworms and boll weevils. For example, see Chem. and Engineering News, May 27, 1985, pp. 46-51 and U.S. Dep. Agric., Agric. Rev. Man., ARM-NE-4. Neem seed extract is considered to be a broad-spectrum insecticide.
While neem seed extract has been shown to be active as an insecticide, it has not come into common use because of the known instability of the formulated extract. Moreover, undesirable aflatoxins, arising from fungal infection of the seed, may be present in the extracts.
Although laboratory scale procedures resulting in extract preparations containing over 25% azadirachtin have been described in the literature, a practical, economical and scalable method has been lacking.
Prior processes for obtaining crude neem extracts typically comprised the steps of:
a) extracting the ground seed with a polar organic solvent such as methanol or ethanol followed by filtration, PA1 b) removing the solvent from the filtrate by evaporation under reduced pressure to yield a dry extract, PA1 c) dissolving the dry extract in a mixture of a water-immiscible polar organic solvent, such as ethyl acetate, and aqueous saturated sodium chloride solution and PA1 d) separating the organic layer, drying and evaporating the solvent to yield the semi-pure extract. PA1 a) deoiling ground neem kernels by stirring with hexane followed by filtration to obtain a deoiled cake or, alternatively, by mechanically extruding the oil and optionally extracting the oil residue with hexane to obtain a deoiled cake, PA1 b) extracting the deoiled cake with methanol followed by filtration, PA1 c) removing the methanol to obtain a dry extract, PA1 d) dissolving about 3 parts of the dry extract in about 7 parts of methanol on a weight/weight (w/w) basis, PA1 e) Adding water preferably with stirring to a final ratio of about 35/65 methanol/water on a volume/volume (v/v) basis, PA1 f) Separating the aqueous methanolic phase from any solid impurities, PA1 g) Diluting the aqueous methanolic mixture with a saturated aqueous salt solution, preferably a sodium chloride solution, PA1 h) Extracting the dilute solution with a water immiscible solvent, such as ethyl acetate, and PA1 i) Drying the organic solution and, if desired, removing the solvent. PA1 a) Evaporating a solution of crude neem extract in methanol obtained in Step b above to a concentrate containing over 65% volatiles, PA1 b) Adding water dropwise with stirring to a final ratio of between 5/95 methanol/water (v/v)to 45/55 methanol/water (v/v), preferably to a final ratio of 35/65 methanol/water (v/v), and PA1 c) Treating the resulting solution as described above by Steps f-i.
Such prior processes result in extracts containing a significant portion of hydrophobic organic impurities and no more than 10% azadirachtin. Higher purity preparations have been described but they require uneconomical, non-scalable chromatographic steps subsequent to those steps described above.
Several attempts have been made by various workers with the objective of removing the hydrophobic materials without resorting to chromatography. For example, Journal of Liquid Chromatography, 10 (6), 1151 (1987) shows an attempt to purify a crude methanolic extract by dissolving the extract in 50% aqueous methanol and then extracting the resulting solution with hexane. Other authors have described the removal of oil from the ground seed with a non-polar solvent such as hexane prior to the polar solvent extraction. However, these methods are not efficient since the crude methanolic or ethanolic extracts, apart from hexane soluble oils, also contain large quantities of hydrophobic impurities which are only sparingly soluble in non-polar solvents such as hexane.
In U.S. Pat. No. 4,556,562, an aqueous storage-stable neem seed extract composition is disclosed. In U.S. Pat. No. 4,946,681, a method is disclosed for stabilizing an alcoholic neem seed extract by removing water with molecular sieves.
Notwithstanding the above advances, there remains a need for an economical, stable composition effective to control pests, preferably a composition which is also free of aflatoxins.